The present invention relates to liquid developer electrostatic photocopying and more particularly to a method of stabilizing charge director solutions and a new stabilized charge director composition.
Processes for forming electrostatic images, existing as electrostatic charge patterns upon a substrate, are well known. In electrostatic printing or copying, a photoconductive imaging surface is first provided with a uniform electrostatic charge, typically by moving the imaging surface past a charge corona at a uniform velocity. The imaging surface is then exposed to an optical image of an original to be copied. This optical image selectively discharges the imaging surface in a pattern to form a latent electrostatic image. In the case of an original bearing dark print on a light background, this latent image consists of substantially undischarged "print" portions corresponding to the graphic matter on the original, admist a "background" portion that has been substantially discharged by exposure to light. The latent image is developed by exposure to oppositely charged, pigmented, toner particles, which deposit on the print portions of the latent image in a pattern corresponding to that of the original.
In liquid developer photocopiers these charged toner particles are suspended in a liquid developer comprising a carrier liquid, toner particles and charge directors. The entire latent electrostatic image is covered with a thin film of liquid developer from a liquid developer reservoir. The charged toner particles in the liquid developer migrate to the oppositely charged "print" portions of the latent image to form a pattern on the photoconductive surface. This pattern, and the corresponding toner particles, are then transferred to a sheet to produce a visible image. Any liquid developer remaining on the photoconductive surface after this process is recycled back into the liquid developer reservoir.
Charge director plays an important role in the developing process described above. The charge director is a chemical species, either molecular or ionic, which acts to control the polarity and charge on the toner particles. The charge director creates charged species causing charging of the imaging material to ensure that the toner particles will be deposited and migrate in such a way as to form the desired image on the imaging surface. Counter ions are also created to keep the liquid developer substantially electrically neutral overall. The present invention may be practiced with any number of charge directors, of which lecithin and barium petronate are examples.
One of the major problems concerning the material used as charge directors is the degradation of the charge carrying species under the application of the electric field created during the electrophoretic development process. Degradation of the charge carrying species also occurs during replenishment of developer with carrier liquid due to dilution of the charge director. Degradation of the charge carrying species destabilizes the liquid developer electrically. Since stable electrical characteristics of the liquid developer are important to achieve a high quality image, particularly when a large number of impressions are to be produced without changing the liquid developer dispersion, degradation of the charge carrying species results in poor copy quality.
It is believed that in many liquid developers the charge director molecules form inverse micelles. An example of these micelles is shown in FIG. 1. The micelles are formed by aggregation such that the polar portion of the charge director molecules point inside, and the nonpolar portion point outside to decrease the overall surface energy of the system. These micelles may solubilize ions generated by the dissociation of the charge director molecules. It is believed that the solubilization of ions by the charge director micelles is due to the formation within and around the micelles, of a microenvironment having a higher dielectric constant. The solubilization of ions by the charge director micelles results in micelles containing a charged species in their center. Some of the micelles have a positive species in the center and others have a negative species in the center. We believe that during the electrophoretic developing process these micelles rupture under the influence of the electric field created by the charged photoconductive surface. The exact mechanism of the rupturing is not known. The rupture of the micelles changes the electrical properties of the liquid developer solution by freeing the charged species in the center of the micelles which, due to their relatively strong positive and negative charges and the low dielectric constant of the carrier liquid, tend to reassociate with each other to form electrically neutral compounds. The formation of these electrically neutral compounds changes the overall electrical properties of the liquid developer. The change in electrical properties of the liquid developer changes the toner particle dispersion in the liquid developer and the number of the charge carrying species resulting in a degradation in copy quality.
We also believe that the micelles rupture when the liquid developer dispersion in a photocopier is replenished by the addition of new carrier liquid. Again, the exact mechanism is not known. The effect of this rupturing is manifested in an instability of the charge carrying species in the system. Again the overall result is a degradation in copy quality.
Accordingly, one object of the present invention is a charge director composition which will resist degradation under the influence of an electric field.
Another object of the present invention is a charge director composition which will resist degradation during the replenishment of carrier liquid in a liquid developer dispersion.
A further object of the present invention is a charge director solution which will resist destabilization.